Cytoskeleton Flashcards
What is the real version of the cell?
The cell is a complex system controlled on a sub-second time scale over sub- to thousands of microns
The cytoskeleton is…
The skeleton and muscles of cells
Define the cytoskeleton
A system of protein polymers that allow:
- architecture
- shape
- motility of cells
- directed movement of organelles and molecules in the cell
Components of the cytoskeleton
Microtubules
Microfilaments (actin filaments)
Intermediate filaments
Host of accessory and regulatory proteins
Polymers of tubulin dimers made up _____ and _____
Polymers of tubulin dimers made up alpha-tubulin and beta-tubulin
Tubulin is what kind of enzyme
A GTPase
Is tubulin solid or hollow and describe more
Hollow with outer diameter of 24 nm
What proteins stabilize and space the polymers and regulate interactions between cytoskeletal elements?
Accessory proteins called microtubule-associated proteins (MAPs)
What are properties of microtubules?
Highly dynamic if not stabilized (can undergo rapid bouts of assembly and disassembly)
Act as substrate for microtubule-based proteins to transport cargo (organelles)
Describe the structure of microtubules
Polarized polymer with a plus and minus end
Plus end = dynamic (lengthens and shortens)
Motor proteins read the polarity and go to one end or the other
Functions of microtubules are…
- Make up the mitotic spindle
- Provide railways for organelle transport in most interphase cells
- Key determinant of cell shape
- Important in neurons for cell shape and axonal transport
- Provide the backbone for cilia and flagella
How many protofilaments are microtubules made of?
13 (+_)
What are protofilaments?
Stacks of tubulin dimers
What end is dynein directed towards?
Minus end
What end is kinesin directed towards?
+ end
Microtubule Associated Proteins are
Structural non-motor proteins
What are the functions of MAPs?
Organize MTs
Regulate MTs stability
Regulate MT dynamics
What is nucleation?
Genesis of a cytoskeletal polymer
Polymerization
The elongation of a cytoskeletal polymer following the initial nucleation
Nucleation for MTs occurs where?
Centrosome
Recent research indicates that golgi fragments may also nucleate MTs
Gamma tubulin in centrosome is only required for what?
Nucleation
The basal body is what?
A microtubule organizing center for cilia and flagella
What is a 9+2 structure of cilia and flagella?
The axoneme
What drives axonemal motility?
Dynein
Immotile Cilia Syndrome
A body-wide defect in axonemal structure that result in obstructive lung disease and sterile males
Kartagener’s Syndrome
A combination of Situs inversus (reversal of normal body asymmetry) and Immotile Cilia Syndrome
Cancer
Target for therapeutics, e.g. disruption of MT dynamics to block cell division (e.g., Taxol)
Diseases associated with malfunction of microtubule based cellular structures
Immotile Cilia Syndrome
Kartagener’s syndrome
Cancer
Lissencephaly
Charcot-Marie-Tooth disease type 2A
Neurodegenerative disorders
Lissencephaly
Mutations in microtubule proteins LIS1 and doublecortin
Charcot-Marie-Tooth disease type 2A
Mutation in specific kinesin has been reported in one case
What occurs in neurodegenerative disorders related to cytoskeleton?
Abnormalities/mutations in tau, dynein, kinesin, spastin
What do neurotopic viruses do?
Exploit neuron’s microtubule based transport system to reach cell bodies
Microfilaments are
Non-hollow polymers of the globular protein actin
Actin is what type of enzyme?
ATPase
What shape is actin?
Helical
Diameter of actin
Roughly 7 nm
Properties of actin
- Highly dynamic if not stabilized; undergo rapid bouts of assembly and disassembly
- Variety of configurations regulated by accessory proteins
- Substrate for members of the myosin family of motors to transport cargo
- Unlike microtubules, actin filaments don’t have specific organizing centers like centrosome and can be nucleated almost anywhere in the cell
Structure of microfilaments (describe both ends)
Polarized filament with barbed end and pointed end
For actin which end is favoured for assembly?
Barbed end
Myosin and polarity of microfilaments
Myosin motors read the polarity and move toward one end or the other (most move toward + end)
Functions of microfilaments
- Concentrated in cell cortex for various functions
- Cleavage furrow for pinching off cells in final stages of mitosis
- Cell motility
- Short-range organelle transport
- Contractility (in both muscle and non-muscle)
What are 3 actin isoforms?
Alpha, beta, and gamma
Alpha actin is _____ _____
Muscle specific
Beta and gamma are found in ___ ____
Most cells
For nucleation and polymerization, actin must be…
ATP loaded
Myosins
F-actin associated force generating mechanoenzymes with roles in contractility and intracellular transport
Most myosins move toward which end of filaments?
barbed end
Microvilli
Actin based epithelial projections
F-actin and myosin II are located at the ___ ___ during cell division
contractile ring
F-actin serves as ____ for spectrin web
scaffold
Diseases association with malfunction of actin based cellular structures
- Hereditary spherocytosis
- Hereditary elliptocytosis
- Breast cancer
- Familial hypertropic cardiomyopathy
- Mutations in skeletal muscle actin
Hereditary spherocytosis
Deforms RBCs to fragile spherocytes bc of weakened binding affinity of spectrin to band 4.1
Hereditary elliptocytosis
Deforms red cells to fragile elliptocytes bc of incomplete formation of spectrin
Breast cancer
In some forms, actin associated protein Tensin, which links integrin receptors to actin cytoskeleton, is disrupted promoting metastatic migration of cancerous cells
Mutations in skeletal muscle actin
Associated with congenital myopathies characterized by structural abnormalities of the muscle and variable degrees of muscle weakness
Myosin VI mutations
Cause deafness
Myosin VII mutations
Associated with deafness, neurological disorder, and blindness
Pathogens take the endogenous actin machinery to…
Translocate within cells
Phalloidin from amanita phalloides (fungi)
Binds to and stabilizes actin filaments
Intermediate filaments structure
Non-polarized
Roughly 10 nm in diameter
Comparatively non-dynamic - much more stable
Functions of intermediate filaments
- Space-filling elements
- Give tensile strength
- Specialized functions depending on cell type
- Important at cell junctions
Intermediate filaments are much _____ _____ across cell types than microtubules or microfilaments
less conserved
Can different cells have different intermediate filaments? What can they be used for?
Yes and can be used as cell-specific “markers”
Diseases associated with malfunction of intermediate filament based cellular structures
- Epidermolysis bullosa simplex
- Progeria
Epidermolysis bullosa simplex
Mutation in keratin genes expressed in basal cell layer of epidermis, result in a skin that is very sensitive to mechanical injury
Progeria
‘Fast aging disease’ associated with a mutation in nuclear lamin protein
How many distinct diseases have been associated with the intermediate filament gene family?
76
Keratins are mutated in more than __ diseases
20
Desmin is mutated in _________
cardiomyopathies
Neurofilament proteins
Small subunit mutated in the IF/2E forms of Charcot-Marie Tooth Disease
Periphin
Induced after peripheral nerve injury
Glial Fibrillary Acidic Protein
Mutated in Alexander disease (leukodystrophy resulting in abnormal myelin)
Arp2/3 binds to…
Pre-existing filament
Describe parts of microvilli
- Barbed end always found at tip
- Actin filaments
- Lateral arm (myosin I-calmodulin): myosin I used
as structural protein - Villin fimbrin: make sure filaments are nicely
arranged - Terminal web at bottom
Describe structures of Myosin I, II, and V
Myosin I: has calmodulin light chains (monomeric so only 1 heavy chain)
Myosin II: has regulatory light chain and essential light chain (dimeric)
Myosin V: has calmodulin light chains (dimeric)
Formins and Spire make what?
Individuals filaments - do not need prior filaments
Give 2 examples of pathogen using endogenous actin machinery to translocate within cells
Listeria monocytogenes infection
Shigella flexneri infection
List 3 leading edge meshworks
Arp 2/3
Profilin
Cofilin
List 3 stress fibers
Myosin
Alpha-actinin
Formins
Periphery actin filaments are nucleated by…
Arp2/3
Center of cell nucleated by…
Formins
The ability of cell to migrate is dependent on what?
Actin cytoskeleton
There is a physical link between the actin cytoskeleton and…
The extracellular environment
Describe kinesin structure
Light chains, heavy chains, coiled coil, and head domain
Describe dynein structure
Light, heavy, intermediate chains, and head domain
Give an example of a neurotopic virus
Herpes virus
What do light and intermediate chains of dynein do?
Regulate function & control what it’s bound to
What are 2 MAPs?
Kinesin and dynein
Where is mucociliary escalator found?
Along airways
Dynein allows for what in MTs?
Allows MTs to bend by sliding them relative to one another
What does cancer drug Taxol do to MTs?
Binds MTs and stabilizes so they no longer have dynamic instability
Rabies virus binds dynein or kinesin?
Dynein when going to minus end where cell body is then goes on kinesin to go to nerve ending to release particles and cycle repeats
Why do myosin VI mutations cause deafness?
Prevent formation of the proper structure of stereocilia in inner ear
What is basic subunit of intermediate filament?
Tetramer
